All machines have switches on
For example, when you have the Coffee Machine, it is normally set to produce Café au Lait using Cow’s Milk. When you click the switch, it gives you the options to produce Espresso, using Dew Drops, or Ice Coffee using Frozen Dew Drops.
About choosing the ingredients
If you try to select a product which requires ingredients that you don’t have in stock, the game will tell you that you don’t have the ingredient in Stock.